1. Field of the Invention
The present invention relates to an image forming apparatus including a light-beam generating unit such as a laser diode, and more particularly, to an image forming apparatus and an abnormality detecting method for detecting anomalous light emission of the light-beam generating unit to perform error processing.
2. Description of the Related Art
An image forming apparatus including a light-beam generating unit has an optical writing device or an optical scanning device including a light-beam generating unit, a light-emission control unit that drives the light-beam generating unit with a driving signal modulated by an image signal input to cause the light-beam generating unit to emit light, a light deflector (a polygon mirror) that deflects and reflects a light beam modulated by an image signal, and an image carrier (a photosensitive drum, etc.) on which a latent image is written according to scanning of the light beam deflected by the light deflector. As the light-beam generating unit, in general, a laser light source such as a laser diode is used. In a color image forming apparatus, for example, color image signals of four colors of yellow (Y), magenta (M), cyan (C), and black (K) modulate light beams, respectively, to form images of Y, M, C, and K on four image carriers. Latent images written on the image carriers are developed by toners, transferred onto a transfer member such as recording paper, and superimposed one on top of another to form a color image.
A driving-current/laser-beam-output characteristic of the laser diode is described. FIG. 16 is a graph of the driving-current/laser-beam-output characteristic of the laser diode. As the driving-current/laser-beam-output characteristic of the laser diode, as shown in FIG. 16, an optical output is weak until a forward current reaches a fixed threshold current Ith and suddenly increases when the forward current exceeds the threshold current Ith. In the image forming apparatus, an electric current obtained by adding an electric current Idr (=light emission current Iη+offset amount) corresponding to an image signal to a bias current Ibi, which is obtained by adding the offset amount to or subtracting the offset amount from the threshold current Ith, is applied to the laser diode as a driving current Iop. The laser diode is driven with the bias current Ibi when an image signal is not input. The laser diode is driven with the driving current Iop=Ibi+Iη+offset amount when an image signal is input. It is possible to drive the laser diode at high speed by always feeding a bias current in this way. Whether the bias current Ibi is set larger than the threshold current Ith depends on a design idea of an image forming engine, in particular, a photosensitive member. When the bias current Ibi is set larger than the threshold current Ith, a rising characteristic of the laser diode is improved. When the bias current Ibi is set smaller than the threshold current Ith, it is possible to prevent scumming while maintaining a certain degree of rising characteristic. In FIG. 16, the bias current Ibi is set smaller than the threshold current Ith.
This driving-current/laser-beam-output characteristic has temperature dependency. As temperature rises, the threshold current Ith tends to increase. An image forming apparatus that includes a bias-current setting unit to make it possible to compensate for the temperature dependency of the threshold current Ith is disclosed in, for example, Japanese Patent Application Laid-Open No. H5-236226. The bias-current setting unit gradually increases the bias current Ibi applied to a laser diode at the time of image formation, detects a laser beam output to generate a monitor signal, and sets the bias current Ibi at the time of image formation based on a bias current value at the time when a level of the monitor signal is higher than a predetermined level.
However, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. H5-236226, when the bias-current setting unit malfunctions because of an influence of noise or the like at the time of setting of the bias current Ibi and the bias current Ibi is set excessively larger than the threshold current Ith shown in FIG. 16, the laser diode emits light even when an image signal is not input to the laser diode. Therefore, an abnormal image such as an image with lateral lines or an entirely solid image is formed in an area without an image. As a result, an image carrier suffers light-induced fatigue and a toner is wasted to impose a burden on a user and adversely affect the image forming apparatus.